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Determination of sulfide with chloranilic acid by biamperometric and automatic potentiometric end-point detection with a lead chloranilate selective electrode
Tulonra,Vol. 39, No. 8, pp. 1007-1011,1992 Printed in Great Britain
00399140/92s5.00 + 0.00 Pergamon Press Ltd
DETERMINATION OF SULFIDE WITH CHLORANILIC ACID BY BIAMPEROMETRIC AND AUTOMATIC POTENTIOMETRIC END-POINT DETECTION WITH A LEAD CHLORANILATE SELECTIVE ELECTRODE J. UBA,
M. A. MALLEA, S. QUINTAR and V. A. CQRTINEZ
Department of Analytical Chemistry, Faculty of Chemistry, Biochemistry and Pharmacy, National University of San Luis, San Luis 5700, Argentina (Received 30 March 1990. Revised 9 December 1991. Accepted 18 December 1991) Sunm~ary-A titrimetric method for determination of sulfide and of sulphur in steels with chloranilic acid by biamperometric and automatic potentiometric end-point detection is described. The construction of the sensor for potentiometric indication is also described. The results obtained agree with those of the iodine-thiosulfate method and with the certified values for the steels.
Determination of sulfide in various natural and manufactured materials is important. As the critical quantities of sulfide are generally small (trace level), sticiently sensitive methods must be used. Classical titrimetry uses iodine or potassium hexacyanoferrate(II1) as the titrant.’ The Methylene Blue method’ is most often used for spectrophotometric determination. The instrumental techniques for rapid determination of sulfur in steels and alloys involve oxidation to sulfur dioxide in a current of oxygen and determination of this product by gas chromatography, spectrophotometry, coulometry or conductimetry.3 Sulfide has also been determined by an automatic potentiometric procedure4 and in MnS and FeS inclusions in steels by absorbing in alkali the hydrogen sulfide evolved on acidification, followed by voltammetric analysis of the solution.5 Recently, potentiometric titration, with ionselective indicator electrodes, has been used for sulfur determination in air and steel samples’ and in sulfur, polysulfide and thiosulfate mixtures.’ Chloranilic acid (2,5-dichloro-3,6-dihydroxyp -benzoquinone) forms sparingly soluble complexes with metal ions such as Pb(II), Bi(III), Hg(I1) and Hg(I), and has been used in precipitation titrations with biamperometric indication for the determination of these ions and for phosphate determination.“2 This paper describes an indirect method for determining sulfide in sulfide solutions and sul-
fur in steels. It is based on the classical evolution procedure,” with absorption of hydrogen sulfide in excess of standard hydroxoplumbate(I1) solution, the surplus Pb(II) being determined by titration with chloranilic acid. Biamperometric and automatic potentiometric indication is used for end-point detection, the first giving high accuracy, and the second considerably shortening the determination time. The detector is a solid-state ion-selective electrode, constructed by impregnating graphite with lead chloranilate by a procedure similar to that described by Hassan and Habib.i4 EXPERIMENTAL
Apparatus
Biamperometric measurements were made by use of a conventional polarization source, with a digital voltmeter and microammeter. A potential difference of 1.OOV was applied across the electrodes. A twin Metrohm (60308000) electrode was used as current sensor. The titrant was added from a lo-ml Metrohm E 485 buret. The sample solution was stirred with a Metrohm G 1514-220 magnetic stirrer. The potentiometric measurements were made with an Orion ionalyzer Model 701 and Orion Automatic Titrator (Automatic System 960), by using the graphite+lead chloranilate electrode in conjunction with a double-junction reference electrode (Orion 90-02). When the Orion Ionalyzer was used, the reagent was added with a Metrohm EA485 multiburet.
1007
1008
J.
bBA
In the determination of sulfur in steels, the basic equipment for the evolution methodi was used, modified by replacing the absorption flask with a long glass tube (400 mm x 7 mm) with delivery tube penetrating far enough into the absorption solution to avoid loss of hydrogen sulfide (Fig. 1). Reagents
All reagents used were of analytical grade. Doubly distilled water was used throughout. No differences in behavior were found between commercially available chloranilic acid and a product purified by recrystallization or sublimation.‘5,‘6 Chloranilic acid solutions in distilled water were standardized by amperometric titration with standard lead(I1) solution.” The stock solution was stored in a darkened container at 1-5”. Periodic checks showed that the solutions could be preserved in this way for a long period of time. Lead chloranilate was prepared by modification of a method for the preparation of barium chloranilate,“’ 1000 ml of O.lM lead nitrate were placed in a 2000-ml beaker and O.OlM chloranilic acid was added from a separating funnel at a rate of 40-60 drops/min, with continual magnetic stirring. The precipitate was left in contact with the supernatant liquid overnight, then centrifuged and washed with water until the purple-red color of the chloranilic acid had disappeared. The product was dried in a vacuum oven at 120” for 12 hr. Preparation of the lead chloranilate electrode
Two spectrographic graphite rods (30 mm x 50 mm) were treated as follows: (a) a suspension
et al.
of lead chloranilate in glacial acetic acid was drawn into one of the rods by suction (Electrode I). (b) O.OlM lead nitrate solution was first drawn into the other rod by suction, followed by O.OlM chloranilic acid to form a precipitate inside the pores and channels of the graphite. Both rods were then dried in a vacuum oven at 120” overnight. The rods were inserted into glass tubes, with 20 mm of the rod exposed to constitute the active surface. The ends of the rods within the tubes were connected to a copper wire (1 mm o.d.) for external contact. The surface between the glass and the graphite was insulated with a waterproof material (SILO@). Procedures Determination of sulfide. Fifty milliliters of N 0.00 1M sodium sulfide solution were added to a mixture of 10 ml of O.OlM lead nitrate treated with enough 2M sodium hydroxide to turn all the Pb(I1) into hydroxoplumbate(I1) (pH 13), and the mixture was stirred magnetically for 15 min. The precipitate was filtered off on a medium texture paper, and washed 3 or 4 times with small quantities of 0.3M sodium hydroxide. The filtrate and washings were evaporated to N 30 ml, cooled, adjusted to 0.1 M acidity with 4M nitric acid, and titrated with O.OlM chloranilic acid. Since aqueous sulfide solutions are unstable, all the samples were treated with the hydroxoplumbate(I1) solution at the same time. S&r in steels. A sample (between 1 and 2.5 g in weight, depending on the sulfur content) was placed in the Erlenmeyer flask and 100 ml of 6M hydrochloric acid (3M acid for the higher sample weights) were added from a separating funnel. The absorption tube was loaded with 5-10 ml (depending on the amount of sulfur) of O.OlM lead nitrate, distilled water, and enough 2M sodium hydroxide to give pH 13 (total volume -30 ml). The Erlenmeyer flask was heated gently and the hydrogen sulfide evolved was absorbed in the hydroxoplumbate(I1) solution. When dissolution of the steel was complete, the delivery tube was disconnected and rinsed with 2 or 3 small volumes of 0.3M sodium hydroxide. The sulphide was then titrated as described above. RESULTS AND DISCUSSION
Fig. 1. Apparatus for determination of sulfur by a modification of the evolution method. (1) 250-ml Erlemneyer flask; (2) W-ml separatory funnel; (3) 50-ml collector tube; (4) delivery tube.
Lead chloranilate electrodes Potential drift and response time. It was ob-
served that in manual titrations
the electrode
Determination
of sulfide with chloranilic acid
attained a stable potential in 5-10 min. Reagents were added every 30 set throughout the titration, except near the end-point, where additions were made every 2 min since stable potential values were attained in this time. The total time for each titration was 20 min. The behavior of the electrode in a standard titration is shown in Fig. 2. Both electrodes tested gave a large potential break at the end-point, but electrode I gave the larger and therefore was used for further tests. Efici of acidity. The optimum acidity range was O.OlM nitric acid, in agreement with that observed in titrations with biamperometric indication.’ At acid concentrations higher than 0. IM the solubility of lead chloranilate increases considerably, but when the acid concentration is lower than O.OlM the electrochemical reaction of chloranilate is inhibited by the poor buffer capacity at the electrode-solution interfaceI The sensitivity of the electrode to changes in acid concentration is shown in Fig. 3. Range of determinable concentrations. The sensor gave a good response in the range from 3 x lo-‘M to 1 x 10e4M lead (II). For concentrations lower than 1 x 10e4M the end-point potential break is much reduced, and the solubility of chloranilic acid becomes the limiting factor for determining lead concentrations higher than 3 x lo-‘M.
0.01 M chloranilic acid, ml Fig. 2. Response of electrodes I and II in the titration of 25 ml of -0.OOlOM lead(I1) with chloranilic acid at pH = 1.0.
1
1009
2
3
4
0.01 M chloranilic acid, ml Fig. 3. Titrations of 25 ml of -0.OOlOM lead(U) with chloranilic acid at various nitric acid concentrations, with use of electrode I.
Accuracy and precision. Twelve titrations of -0.00144 lead nitrate in O.lM nitric acid with O.OlM chloranilic acid gave a mean of 0.207 g/l. lead, with a standard deviation of 0.0007 g/l. The relative error was estimated by comparison of the values obtained by potentiometric indication with those corresponding to the average of three titrations performed with biamperometric indication, and was about 2%. The behavior of the sensor in automatic titrations was checked with nine repetitive analyses of another lead nitrate solution; the mean found was 0.223 g/l.lead, standard deviation 0.008 g/l. Titration with biamperometric indication gave 0.219 g/l. (mean of three titrations). Analysis of surfide solutions. Table 1 shows the results of six replicate biamperometric and automatic potentiometric titrations of 25 ml of nominally 0.030 g/l. sodium sulfide solution with O.OlM chloranilic acid and of titrations by the iodine-thiosulfate method recommended by Budd and Bewick’ for standardization of sodium sulfide solutions in the Methylene Blue method. The displacement reaction between mercury chloranilate and sulfide, used by Hoffmann for absorptiometric sulfide determination,20 were found not to be suitable for titrations. The pH of the Pb(I1) solution was chosen as - 13 because at a pH lower than 7 sulfide was
J. UA
1010
et al.
Table 1. Comparison of results obtained in the analysis of 25 ml of 0.030 gjl. sulfide solution by different methods (n = 6)
0.0296 0.0009
Mean, g/l. St. devn., g/l.
Potentiometric method
Biamperometric method
Iodine-thiosulfate method
0.0298 0.0010
0.0297 0.0015
Table 2. Comparison of results obtained in the determination of sulphur in three certified steels samples by the biamperometric and potentiometric methods (n = 6) Sample weight, g Certified S value, % S found f std. devn., % Biamperometric method Potentiometric method
IPT- 14A
IPT 43
IPT 37
2.000 0.036
1.000 0.112
2.500 0.082
0.0357 f 0.0016 0.0356 f 0.0016
0.112 f 0.007 0.111 *o.Oo
0.0813 f 0.0020 0.0817 It 0.0012
only partially retained, and between pH 7 and 12.5 lead hydroxide is precipitated. Steel analyses Three certified samples from the “Instituto de Pesquisas Tecnologicas” (IPT), San Pablo, Brazil: IPT No. 43 (AISI 1132), IPT No. 14A (AISI 1040) and IPT No. 37 grey iron were analyzed for sulfur content. The results are shown in Table 2. It was verified that the optimum weight of sample for steels containing about 0.1% of sulfur was about 1 g. With higher weights of sample (e.g., 2.5 g), the evolution of hydrogen sulfide was very fast, causing considerable losses. Also, prolonged heating was necessary to complete the dissolution, and some hydrochloric acid distilled into the absorption tube. Lower weights of sample allowed more dilute acid (3M) to be used, which considerably reduced this risk. When the sulfur content is lower, as in the IPT standards 14A and 37, it is more convenient to use higher weights of sample (2-2.5 g) in order to decrease the amount of lead chlorani-
late precipitate, which disturbs the end-point location by either detection mode. The procedure proposed shows several advantages over the classical evolution method. (a) It is more exact and precise since it diminishes the risk of hydrochloric acid distilling into the absorption solution (because it requires a lower weight of sample, which also reduces the dissolution time). (b) The electrochemical behavior of chloranilic acid at E = 1.00 V allows amperometric curves to be obtained with a great number of points in both branches (Fig. 4). This means that the determination of the end-point can be made more accurate by applying the least-squares method. (c) When automatic potentiometric titration is used, the time required for the determination is mainly that consumed in the dissolution of the sample and conditioning of the solution for the titration. (d) The potentiometric sensor developed is strong, very low in cost, easily manufactured, long-lasting and very easily reactivated. Its optimum acidity range allows it to be used in solutions where the use of other sensors, such as the Orion 94-82-00 lead electrode,*’ is impossible. Acknowledgements-The authors wish to thank the Consejo National de Investigaciones Cientificas y T&micas (CONICET) for financial support of this research, and Lit. Maria Estela Lopez for linguistic advice.
REFERENCES
l/ +._._.._._ -.-*_ ..___+. 0
2
4
6
ml Chloranilic acid Fig. 4. Amperometric titration curve (applied e.m.f. 1.00 V).
L. A. Haddock, in C. L. Wilson and D. W. Wilson (cds.), Comprehensive Analytical Chemistry, Vol. Ic, p. 282. Elsevier, Amsterdam. 1962. M. Budd and H. A. Bewick, Anal. Chem., 1952,24,158. B. Nebesar, J. Chem. Educ., 1971, 48, A751. B. A. Furmanov, L. N. Strebkova and 0. L. Cherkasskaya, Bum. Prom-s& 1985, No.1, 18; Chem. Abstr., 1985, 102, 115385~.
Determination
of sulfide with chloranilic acid
5. I. Zezula, Hum. Ltsty, 1985, 40, 197; Chem. Abstr., 1985, 102, 170476b. 6. A. C. Calokerinos, M. Timotheou-Potamia, E. Sarantonis and T. P. Hadjiioannou, Anal. Chim. Actu, 1983, 151, 85. 7. T. D. Gornostaeva, V. A. Pronin and V. Ya. Semenov, Zaoodrk Lab., 1983, 49, No.7, 14; Chem. Abstr., 1983, 99, 132949~. 8. V. A. Cortinea and C. B. Marone, Quim. Anal., 1976, 30, 33; Chem. Abstr., 1977, 86, 164752~. 9. V. A. Cortinez, 0. Baudino and C. B. Marone, Afinidad, 1979, 36, 495; Chem. Abstr., 1980, 93, 6042lt. 10. 0. M. Baudino, V. A. Cotinez and C. B. Marone, Reo. Asoc. Rtoqutm. Argent., 1976, 41, 212; Chem. Abstr., 1978, 88, 197744s. 11. M. A. Mallea, H. Marini, V. A. Cortinez and C. B. Marone, Cron. Chim., 1981, 66, 14. 12. M. A. Mallea, S. Quintar de Guzman and V. A. Cortinez, Talanta, 1989, 36, 792.
1011
13. F. Welcher (cd.), Stan&d Methods of Chemical Analysts, 6th Ed., Vol. HA, p. 677. Van Nostrand, Princeton, 1963. 14. S. S. M. Hassan and M. M. Habib, Anal. Chem., 1981, 53, 508. 15. B. J. Thamer and A. F. Voight, J. Phys. Chem., 1952, 56, 225. 16. D. K. Cabbinesa, E. S. Amis and K. C. Jackson, J. Chem. Eitg. Data, 1967, 12, 90. 17. H. A. Flaschka, Titrations, 2nd Ed., p. 80. Pergamon Press, Oxford, 1964. 18. R. J. Bertolacini and J. E. Barney II, Anal. Chem., 1957, 29, 281. 19. 0. H. Milller, Pohzrography, in A. Weissberger (ed.), Physical Methoa!s of Organic Chemistry, 4th Ed., Vol. I, Part. IV, P. 3155. Interscience, New York, 1060. 20. E. Hoffmann, Z. Anal. Chem., 1962, 185, 372. 21. Orion Research, Analytical Method Guide, 1978.
00399140/92s5.00 + 0.00 Pergamon Press Ltd
DETERMINATION OF SULFIDE WITH CHLORANILIC ACID BY BIAMPEROMETRIC AND AUTOMATIC POTENTIOMETRIC END-POINT DETECTION WITH A LEAD CHLORANILATE SELECTIVE ELECTRODE J. UBA,
M. A. MALLEA, S. QUINTAR and V. A. CQRTINEZ
Department of Analytical Chemistry, Faculty of Chemistry, Biochemistry and Pharmacy, National University of San Luis, San Luis 5700, Argentina (Received 30 March 1990. Revised 9 December 1991. Accepted 18 December 1991) Sunm~ary-A titrimetric method for determination of sulfide and of sulphur in steels with chloranilic acid by biamperometric and automatic potentiometric end-point detection is described. The construction of the sensor for potentiometric indication is also described. The results obtained agree with those of the iodine-thiosulfate method and with the certified values for the steels.
Determination of sulfide in various natural and manufactured materials is important. As the critical quantities of sulfide are generally small (trace level), sticiently sensitive methods must be used. Classical titrimetry uses iodine or potassium hexacyanoferrate(II1) as the titrant.’ The Methylene Blue method’ is most often used for spectrophotometric determination. The instrumental techniques for rapid determination of sulfur in steels and alloys involve oxidation to sulfur dioxide in a current of oxygen and determination of this product by gas chromatography, spectrophotometry, coulometry or conductimetry.3 Sulfide has also been determined by an automatic potentiometric procedure4 and in MnS and FeS inclusions in steels by absorbing in alkali the hydrogen sulfide evolved on acidification, followed by voltammetric analysis of the solution.5 Recently, potentiometric titration, with ionselective indicator electrodes, has been used for sulfur determination in air and steel samples’ and in sulfur, polysulfide and thiosulfate mixtures.’ Chloranilic acid (2,5-dichloro-3,6-dihydroxyp -benzoquinone) forms sparingly soluble complexes with metal ions such as Pb(II), Bi(III), Hg(I1) and Hg(I), and has been used in precipitation titrations with biamperometric indication for the determination of these ions and for phosphate determination.“2 This paper describes an indirect method for determining sulfide in sulfide solutions and sul-
fur in steels. It is based on the classical evolution procedure,” with absorption of hydrogen sulfide in excess of standard hydroxoplumbate(I1) solution, the surplus Pb(II) being determined by titration with chloranilic acid. Biamperometric and automatic potentiometric indication is used for end-point detection, the first giving high accuracy, and the second considerably shortening the determination time. The detector is a solid-state ion-selective electrode, constructed by impregnating graphite with lead chloranilate by a procedure similar to that described by Hassan and Habib.i4 EXPERIMENTAL
Apparatus
Biamperometric measurements were made by use of a conventional polarization source, with a digital voltmeter and microammeter. A potential difference of 1.OOV was applied across the electrodes. A twin Metrohm (60308000) electrode was used as current sensor. The titrant was added from a lo-ml Metrohm E 485 buret. The sample solution was stirred with a Metrohm G 1514-220 magnetic stirrer. The potentiometric measurements were made with an Orion ionalyzer Model 701 and Orion Automatic Titrator (Automatic System 960), by using the graphite+lead chloranilate electrode in conjunction with a double-junction reference electrode (Orion 90-02). When the Orion Ionalyzer was used, the reagent was added with a Metrohm EA485 multiburet.
1007
1008
J.
bBA
In the determination of sulfur in steels, the basic equipment for the evolution methodi was used, modified by replacing the absorption flask with a long glass tube (400 mm x 7 mm) with delivery tube penetrating far enough into the absorption solution to avoid loss of hydrogen sulfide (Fig. 1). Reagents
All reagents used were of analytical grade. Doubly distilled water was used throughout. No differences in behavior were found between commercially available chloranilic acid and a product purified by recrystallization or sublimation.‘5,‘6 Chloranilic acid solutions in distilled water were standardized by amperometric titration with standard lead(I1) solution.” The stock solution was stored in a darkened container at 1-5”. Periodic checks showed that the solutions could be preserved in this way for a long period of time. Lead chloranilate was prepared by modification of a method for the preparation of barium chloranilate,“’ 1000 ml of O.lM lead nitrate were placed in a 2000-ml beaker and O.OlM chloranilic acid was added from a separating funnel at a rate of 40-60 drops/min, with continual magnetic stirring. The precipitate was left in contact with the supernatant liquid overnight, then centrifuged and washed with water until the purple-red color of the chloranilic acid had disappeared. The product was dried in a vacuum oven at 120” for 12 hr. Preparation of the lead chloranilate electrode
Two spectrographic graphite rods (30 mm x 50 mm) were treated as follows: (a) a suspension
et al.
of lead chloranilate in glacial acetic acid was drawn into one of the rods by suction (Electrode I). (b) O.OlM lead nitrate solution was first drawn into the other rod by suction, followed by O.OlM chloranilic acid to form a precipitate inside the pores and channels of the graphite. Both rods were then dried in a vacuum oven at 120” overnight. The rods were inserted into glass tubes, with 20 mm of the rod exposed to constitute the active surface. The ends of the rods within the tubes were connected to a copper wire (1 mm o.d.) for external contact. The surface between the glass and the graphite was insulated with a waterproof material (SILO@). Procedures Determination of sulfide. Fifty milliliters of N 0.00 1M sodium sulfide solution were added to a mixture of 10 ml of O.OlM lead nitrate treated with enough 2M sodium hydroxide to turn all the Pb(I1) into hydroxoplumbate(I1) (pH 13), and the mixture was stirred magnetically for 15 min. The precipitate was filtered off on a medium texture paper, and washed 3 or 4 times with small quantities of 0.3M sodium hydroxide. The filtrate and washings were evaporated to N 30 ml, cooled, adjusted to 0.1 M acidity with 4M nitric acid, and titrated with O.OlM chloranilic acid. Since aqueous sulfide solutions are unstable, all the samples were treated with the hydroxoplumbate(I1) solution at the same time. S&r in steels. A sample (between 1 and 2.5 g in weight, depending on the sulfur content) was placed in the Erlenmeyer flask and 100 ml of 6M hydrochloric acid (3M acid for the higher sample weights) were added from a separating funnel. The absorption tube was loaded with 5-10 ml (depending on the amount of sulfur) of O.OlM lead nitrate, distilled water, and enough 2M sodium hydroxide to give pH 13 (total volume -30 ml). The Erlenmeyer flask was heated gently and the hydrogen sulfide evolved was absorbed in the hydroxoplumbate(I1) solution. When dissolution of the steel was complete, the delivery tube was disconnected and rinsed with 2 or 3 small volumes of 0.3M sodium hydroxide. The sulphide was then titrated as described above. RESULTS AND DISCUSSION
Fig. 1. Apparatus for determination of sulfur by a modification of the evolution method. (1) 250-ml Erlemneyer flask; (2) W-ml separatory funnel; (3) 50-ml collector tube; (4) delivery tube.
Lead chloranilate electrodes Potential drift and response time. It was ob-
served that in manual titrations
the electrode
Determination
of sulfide with chloranilic acid
attained a stable potential in 5-10 min. Reagents were added every 30 set throughout the titration, except near the end-point, where additions were made every 2 min since stable potential values were attained in this time. The total time for each titration was 20 min. The behavior of the electrode in a standard titration is shown in Fig. 2. Both electrodes tested gave a large potential break at the end-point, but electrode I gave the larger and therefore was used for further tests. Efici of acidity. The optimum acidity range was O.OlM nitric acid, in agreement with that observed in titrations with biamperometric indication.’ At acid concentrations higher than 0. IM the solubility of lead chloranilate increases considerably, but when the acid concentration is lower than O.OlM the electrochemical reaction of chloranilate is inhibited by the poor buffer capacity at the electrode-solution interfaceI The sensitivity of the electrode to changes in acid concentration is shown in Fig. 3. Range of determinable concentrations. The sensor gave a good response in the range from 3 x lo-‘M to 1 x 10e4M lead (II). For concentrations lower than 1 x 10e4M the end-point potential break is much reduced, and the solubility of chloranilic acid becomes the limiting factor for determining lead concentrations higher than 3 x lo-‘M.
0.01 M chloranilic acid, ml Fig. 2. Response of electrodes I and II in the titration of 25 ml of -0.OOlOM lead(I1) with chloranilic acid at pH = 1.0.
1
1009
2
3
4
0.01 M chloranilic acid, ml Fig. 3. Titrations of 25 ml of -0.OOlOM lead(U) with chloranilic acid at various nitric acid concentrations, with use of electrode I.
Accuracy and precision. Twelve titrations of -0.00144 lead nitrate in O.lM nitric acid with O.OlM chloranilic acid gave a mean of 0.207 g/l. lead, with a standard deviation of 0.0007 g/l. The relative error was estimated by comparison of the values obtained by potentiometric indication with those corresponding to the average of three titrations performed with biamperometric indication, and was about 2%. The behavior of the sensor in automatic titrations was checked with nine repetitive analyses of another lead nitrate solution; the mean found was 0.223 g/l.lead, standard deviation 0.008 g/l. Titration with biamperometric indication gave 0.219 g/l. (mean of three titrations). Analysis of surfide solutions. Table 1 shows the results of six replicate biamperometric and automatic potentiometric titrations of 25 ml of nominally 0.030 g/l. sodium sulfide solution with O.OlM chloranilic acid and of titrations by the iodine-thiosulfate method recommended by Budd and Bewick’ for standardization of sodium sulfide solutions in the Methylene Blue method. The displacement reaction between mercury chloranilate and sulfide, used by Hoffmann for absorptiometric sulfide determination,20 were found not to be suitable for titrations. The pH of the Pb(I1) solution was chosen as - 13 because at a pH lower than 7 sulfide was
J. UA
1010
et al.
Table 1. Comparison of results obtained in the analysis of 25 ml of 0.030 gjl. sulfide solution by different methods (n = 6)
0.0296 0.0009
Mean, g/l. St. devn., g/l.
Potentiometric method
Biamperometric method
Iodine-thiosulfate method
0.0298 0.0010
0.0297 0.0015
Table 2. Comparison of results obtained in the determination of sulphur in three certified steels samples by the biamperometric and potentiometric methods (n = 6) Sample weight, g Certified S value, % S found f std. devn., % Biamperometric method Potentiometric method
IPT- 14A
IPT 43
IPT 37
2.000 0.036
1.000 0.112
2.500 0.082
0.0357 f 0.0016 0.0356 f 0.0016
0.112 f 0.007 0.111 *o.Oo
0.0813 f 0.0020 0.0817 It 0.0012
only partially retained, and between pH 7 and 12.5 lead hydroxide is precipitated. Steel analyses Three certified samples from the “Instituto de Pesquisas Tecnologicas” (IPT), San Pablo, Brazil: IPT No. 43 (AISI 1132), IPT No. 14A (AISI 1040) and IPT No. 37 grey iron were analyzed for sulfur content. The results are shown in Table 2. It was verified that the optimum weight of sample for steels containing about 0.1% of sulfur was about 1 g. With higher weights of sample (e.g., 2.5 g), the evolution of hydrogen sulfide was very fast, causing considerable losses. Also, prolonged heating was necessary to complete the dissolution, and some hydrochloric acid distilled into the absorption tube. Lower weights of sample allowed more dilute acid (3M) to be used, which considerably reduced this risk. When the sulfur content is lower, as in the IPT standards 14A and 37, it is more convenient to use higher weights of sample (2-2.5 g) in order to decrease the amount of lead chlorani-
late precipitate, which disturbs the end-point location by either detection mode. The procedure proposed shows several advantages over the classical evolution method. (a) It is more exact and precise since it diminishes the risk of hydrochloric acid distilling into the absorption solution (because it requires a lower weight of sample, which also reduces the dissolution time). (b) The electrochemical behavior of chloranilic acid at E = 1.00 V allows amperometric curves to be obtained with a great number of points in both branches (Fig. 4). This means that the determination of the end-point can be made more accurate by applying the least-squares method. (c) When automatic potentiometric titration is used, the time required for the determination is mainly that consumed in the dissolution of the sample and conditioning of the solution for the titration. (d) The potentiometric sensor developed is strong, very low in cost, easily manufactured, long-lasting and very easily reactivated. Its optimum acidity range allows it to be used in solutions where the use of other sensors, such as the Orion 94-82-00 lead electrode,*’ is impossible. Acknowledgements-The authors wish to thank the Consejo National de Investigaciones Cientificas y T&micas (CONICET) for financial support of this research, and Lit. Maria Estela Lopez for linguistic advice.
REFERENCES
l/ +._._.._._ -.-*_ ..___+. 0
2
4
6
ml Chloranilic acid Fig. 4. Amperometric titration curve (applied e.m.f. 1.00 V).
L. A. Haddock, in C. L. Wilson and D. W. Wilson (cds.), Comprehensive Analytical Chemistry, Vol. Ic, p. 282. Elsevier, Amsterdam. 1962. M. Budd and H. A. Bewick, Anal. Chem., 1952,24,158. B. Nebesar, J. Chem. Educ., 1971, 48, A751. B. A. Furmanov, L. N. Strebkova and 0. L. Cherkasskaya, Bum. Prom-s& 1985, No.1, 18; Chem. Abstr., 1985, 102, 115385~.
Determination
of sulfide with chloranilic acid
5. I. Zezula, Hum. Ltsty, 1985, 40, 197; Chem. Abstr., 1985, 102, 170476b. 6. A. C. Calokerinos, M. Timotheou-Potamia, E. Sarantonis and T. P. Hadjiioannou, Anal. Chim. Actu, 1983, 151, 85. 7. T. D. Gornostaeva, V. A. Pronin and V. Ya. Semenov, Zaoodrk Lab., 1983, 49, No.7, 14; Chem. Abstr., 1983, 99, 132949~. 8. V. A. Cortinea and C. B. Marone, Quim. Anal., 1976, 30, 33; Chem. Abstr., 1977, 86, 164752~. 9. V. A. Cortinez, 0. Baudino and C. B. Marone, Afinidad, 1979, 36, 495; Chem. Abstr., 1980, 93, 6042lt. 10. 0. M. Baudino, V. A. Cotinez and C. B. Marone, Reo. Asoc. Rtoqutm. Argent., 1976, 41, 212; Chem. Abstr., 1978, 88, 197744s. 11. M. A. Mallea, H. Marini, V. A. Cortinez and C. B. Marone, Cron. Chim., 1981, 66, 14. 12. M. A. Mallea, S. Quintar de Guzman and V. A. Cortinez, Talanta, 1989, 36, 792.
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